It is understood from the related art that, for the initiation of the combustion process of an internal combustion engine having externally supplied ignition, one may conduct an electric current through a primary coil having a low number of turns wound onto an iron core. The center conductor of a spark plug is connected to a second coil that is situated on the same iron core and has a larger number of turns. At the time of the intended ignition, the current flow through the primary coil is interrupted. This leads to the induction of a high voltage in the second coil, which discharges in a spark in the air gap of the spark plug.
In order to be able to control the time of the interruption of the current flow, and with that the ignition time, using an engine control device, it is provided in the related art that one should interrupt the current flow using at least one switching transistor. The switching transistor is switched by a control signal, in this instance, which is generated in the engine control unit in a control circuit. In this instance, the ignition time for each individual ignition is determined, for example, using a characteristics map stored in the engine control unit, as a function of the respective operating state of the internal combustion engine.
Frequently it is provided that the engine control, with the control circuit, is situated in the vehicle at a spatially different place from the ignition coil and the transistor assigned for switching the current flow. In this case, a cable connection is provided between the two components. Such a cable connection may have an error during the operation of the vehicle. The cable connection may be interrupted, for example. Besides that, the insulation of the cable connection may be damaged, which mostly leads to a short circuit. Both errors are easy to detect in the control circuit. In the case of an interruption in the cable connection, the electric current transmitted via the cable connection drops below a specifiable boundary value. In the case of a short circuit, the current on the connecting line rises above a specifiable boundary value. If the current flowing over the line is greater than the first boundary value, but lower than the second boundary value, there exists a normal operation of the line connection and the switching transistor. The two boundary values are therefore adjusted to the required control current of the switching transistor.
Starting from this related art, the problem arises that when a switching transistor is exchanged for an equally functioning, but not identical type, a different control current is required. If this control current is greater or less than the boundary values stored in the engine control unit, an error message is generated in the engine control unit, and the operation of the internal combustion engine becomes impossible.
Consequently, the present invention is based on the object of stating a circuit configuration for switching current flow through a coil, which is able to replace a multitude of functionally equal, but not identical circuit configurations, without having an error message generated in the engine control unit.
The object is attained, according to the present invention, by a circuit configuration for switching current flow through an ignition coil, in which the switching process is able to be executed by at least one first transistor that is controllable by a control signal which is able to be supplied via a control signal input of the circuit configuration, and the control signal input is connected to at least one variable resistor.
It is proposed, according to the present invention, to position a component having a variable electrical resistance at the control signal input of the circuit configuration, which is provided to accept a control signal that is generated in the engine control unit. Using this resistance element, the input current of the circuit configuration is able to be adjusted. In this way it is possible to set the input current to a specifiable setpoint value, so that the current supplied by the control circuit is able to be adjusted within limits within which the error monitoring of the control circuit does not report any error operation.
The variable resistance element may be situated both in series with the switching transistor, so that a specifiable voltage value drops off at the resistance element, and in parallel, so that a power loss is shunted to ground via the resistance element. In the one case, the input resistance of the circuit configuration is able to be increased by the variable resistance element beyond the input resistance of the switching transistor, and in the other case, the resistance is able to be reduced.
The switching transistor provided for switching the current flow through the ignition coil may, for instance, be a field-effect transistor or an IGBT. The use of a bipolar transistor is especially preferred. Furthermore, the circuit configuration may have additional components in order, for example, to detect an overvoltage at the ignition coil, an overcurrent or an over temperature of the at least one switching transistor, and in order to implement protective circuits which avoid the failure of these components.
In one refinement of the present invention, a measuring resistor is also provided in order to monitor the current flow through the variable resistance element. Using a regulating device that is occasionally provided, the measured actual value is able to be adjusted to a specifiable setpoint value by adjusting the variable resistance element in such a way that the circuit configuration takes up a specifiable input current. In this way, the input current is able to be held constant, even in response to a changing supply voltage. Changing supply voltages occur in motor vehicles particularly during the starting process, when the battery is stressed by the electrical starter, but the generator does not yet supply any charging voltage.
The variable resistance element may be switched either stepwise, or may be provided for the continuous changing of its resistance value. A continuous change is implementable particularly by using at least one transistor as resistance element. In this context, what may be involved is either the collector-emitter path of a bipolar transistor or the channel region of a field-effect transistor. The resistance of a collector-emitter path is able to be continuously influenced via the basic current applied. The resistance of a channel region is able to be influenced via the gate voltage applied. In order to implement a stepped switchover of the input resistance, at least one resistance element is able to be connected to the control signal input of the circuit configuration using a switch. A field-effect transistor or a bipolar transistor or a DIP switch is particularly suitable as an electric switch.
Preferably, but not necessarily, the circuit configuration proposed by the present invention is located in the same housing as the ignition coil and the ignition transformer. A compact component then comes about, which is able to be protected well from spray water and environmental influences, and which requires only one error-prone cable connection to the engine control unit for transmitting the control signal. The reliability may be increased further if the circuit configuration, proposed according to the present invention, is integrated monolithically on a single semiconductor substrate.
The present invention will be explained in greater detail below, in light of exemplary embodiments, which are partially shown in the attached figures.